1. Field of the Invention
The present invention relates generally to an apparatus and method for generating laser energy, and more particularly, to passively modelocked lasers for providing ultra-short pulses with enhanced stability.
2. State of the Art
Both actively modelocked lasers and passively modelocked lasers are well known in the laser art. For example, modelocked lasers can be formed as ultra-short pulse sources using single-mode rare-earth-doped fibers for providing compact ultra-fast technology. One particularly useful fiber pulse source is based on Kerr-type passive modelocking. Such a pulse source can be assembled using widely available standard fiber components to provide pulses at the bandwidth limit of rare-earth fiber lasers with GigaHertz repetition rates.
Any practical fiber-based ultra-short pulse source must be environmentally stable in order to be commercially practical. As referenced herein, the phrase "environmentally stable" refers to a pulse source which is substantially immune to a loss of pulse generation due to environmental influences such as temperature drifts and which is, at most, only slightly sensitive to pressure variations. On the contrary, conventional fiber-based ultra-short pulse sources are susceptible to both temperature and pressure variations, and require constant monitoring and control by the user to maintain pulse generation.
To address the environmental sensitivity of an ultra-short pulse source cavity, Kerr-type modelocking in a polarization maintaining erbium-doped fiber has been proposed. For example, a document entitled "Passive mode locking by using nonlinear polarization evolution in a polarization-maintaining erbium-doped fiber", Fermann, M. E. et al., OPTICS LETTERS/Vol. 18, No. 11, Jun. 1, 1993, PP. 894-896, the disclosure of which is hereby incorporated by reference, describes Kerr-type modelocking which involves a differential excitation of two linearly polarized fundamental eigenmodes of the polarization maintaining erbium-doped fiber. The two fundamental eigenmodes accumulate a differential non-linear phase delay after a certain propagation distance. Due to interference of the eigenmodes at a polarizer, the non-linear phase delay translates into an amplitude modulation which can provide sufficient pulse-shortening per round-trip to produce passive modelocking.
However, such passive modelocking suffers significant drawbacks. For example, the amplitude modulation is also sensitive to the linear phase delay between the two interfering eigenmodes. Because the linear phase delay is subject to temperature and pressure variations in the polarization maintaining erbium-doped fiber, a continuously adjustable phase control is necessary to ensure stable modelocking operation.
Another attempt to address the instability problems of an ultra-short pulse source cavity uses Kerr-type modelocking in a so-called "figure of eight laser" (i.e., F8L). An F8L is described in a document entitled "Polarization maintaining figure-8 laser", by Taverner, D. et al, presented at the Optical Soc. Am. Top. meeting on non-linear guided wave phenomena at Cambridge, 1993, paper WC No. 3. This document discloses amplitude modulation in a polarization maintaining fiber laser using Kerr-type modelocking. As described therein, use of a phase controller is avoided due to a reciprocal characteristic of the polarization maintaining F8L whereby the linear phase delay along two interfering polarization directions the F8L is always 0.
However, the disclosed F8L suffers significant drawbacks. For example, passive phase modulation is translated into amplitude very inefficiently. Long lengths of polarization maintaining fiber (e.g., greater than 300 meters) are therefore required within the cavity to obtain passive modelocking, and pulses generated by the F8L are relatively long in duration (e.g., greater than 2 picoseconds). Further, the pulses are generated with a relatively small pulse energy (e.g., less than 10 picoJoules) and the F8L is limited to a relatively small stability range. Because highly-birefringent fiber is used in the cavity, accurate alignment of splices between the fiber components is required, thus increasing complexity and cost of the laser assembly.
Another attempt to overcome the environmental sensitivity of the phase delay between the two interfering modes of a Kerr-type modelocked fiber laser cavity (or in this particular case the phase delay between the polarization eigenmodes of the fiber) was recently disclosed by I. N. Duling III and R. D. Esnam in a document entitled "Single-Polarisation Fibre Amplifier," Electronics Letters, Jun. 4, 1992 Vol. 28, No. 12. In this a Faraday rotator mirror is used in conjunction with a polarizer to ensure that the output polarization state of a fiber amplifier is independent of environmental perturbations. As described, the technique is not suitable for the implementation into a Kerr-type modelocked fiber laser for several reasons. 1) The linear phase delay between the polarization eigenmodes is compensated to be exactly zero after one round-trip and cannot be adjusted. 2) The Faraday rotator mirror rotates the polarization state by 90.degree. after one round trip, which results in a complete loss of the signal light at the polarizer, preventing the onset of laser oscillation. 3) Nonlinear polarization evolution is not controlled by this simple method, which is necessary for the operation of a modelocked fiber laser.
Accordingly, it would be desirable to provide an environmentally stable ultra-short pulse source which does not require operator input and/or continuous phase control to maintain stability. Further, it would be desirable to provide such a pulse source in a cost-effective manner to provide a commercially practical, ultra-short pulse source suitable for such uses as communication environments.